Spaced-repetition learning systems (SRS) These are, basically, flash card programs. What makes these programs special is that they decide what cards are to be reviewed every day. It does this based on psychological studies of forgetting. Ideally, one uses the SRS program every day. The program gives you the cards you need to review that day, based on the notion that the best time to review a card is just before you are likely to forget it.

You see the question, try to answer it in your head, and then look at the answer. You indicate to the SRS program if you got the question correct or not, and that feeds information into the algorithm to choose when to show you the card again. If you know a fact very well, it might not ask you for a year or more.